Heretofore, as a coating material to present a coating film excellent in weather resistance, a coating material containing a fluororesin (fluororesin coating material) has been known. By virtue of its high weather resistance, the fluororesin coating material has been used as a top coating to be applied to the topmost surface of various substrates. As such a fluororesin to be used for a top coating, a solvent-soluble one is common which is soluble in an organic solvent such as xylene or cyclohexanone. The solvent-soluble fluororesin coating material has attracted attention as a coating material provided with both high durability and excellent ornamental design properties, whereby excellent glossy finish is possible. However, as compared with a common resin such as an acrylic resin or a polyester resin, the solvent-soluble type fluororesin coating material is inadequate in the pigment dispersibility. Therefore, usually, carboxy groups are introduced into a fluorinated copolymer to improve the pigment dispersibility of such a copolymer.
As a fluororesin for coating material having the pigment dispersibility improved, for example, a fluorinated polymer for coating material has been proposed wherein carboxy groups are introduced into the copolymer by reacting hydroxyl groups present in the copolymer with a dibasic acid (Patent Document 1). Further, as a pigment dispersion for fluororesin coating material having an improved dispersion stability or an improved gloss and mixed color stability of a dried coating film in the case of using it as a raw material for coating material, a pigment dispersion has been proposed which comprises a fluorinated copolymer containing a hydroxyalkyl crotonate monomer, a pigment, a pigment-dispersing agent and an organic solvent (Patent Document 2).
In recent years, for the protection of natural environment, global warming due to discharge of organic solvents and air pollution by e.g. photochemical smog have been regarded as problematic, and emission control has been implemented. Especially in Europe and U.S.A., such control is advanced, and also in Japan, Air Pollution Control Act was revised in 2006, and legal emission control has been initiated.
Therefore, in the field of coating materials, an aqueous coating composition has been developed which contains a synthetic resin and wherein only water or a mixture of water and a water-soluble organic solvent is used as a medium. As the synthetic resin to be dissolved and/or dispersed in the aqueous coating composition, various ones have now been proposed. As one of such synthetic resins, a fluorinated copolymer having carboxy groups bonded thereto is known.
Such an aqueous coating composition is used as a coating material by adding, as the case requires, a colorant such as a pigment, a plasticizer, an ultraviolet absorber, a leveling agent, an anti-cissing agent, an anti-skinning agent, a curing agent, etc.
In a case where a pigment is added to the aqueous coating composition, it is common that together with the pigment, a pigment dispersing agent is employed. In the aqueous coating material, the pigment-stabilizing mechanism is formed by electrostatic repulsion and steric hindrance by the pigment dispersing agent. Generally, the stabilizing degree due to the electrostatic repulsion is proportional to the thickness of electric double layer, and as a dispersing agent showing a strong electrostatic repulsion, a polycarboxylic acid or a polycarboxylate is known. For example, a case where a titanium oxide pigment is dispersed by using Nopcose 44-C (manufactured by SanNopco Limited, the structure is a polycarboxylic acid) as a dispersing agent, to an aqueous dispersion containing a fluorinated polymer having a hydrophilic moiety, obtained by emulsion polymerization (Patent Document 3). Further, a case where a titanium oxide pigment is dispersed by using Disrol H-14N dispersant (manufactured by Nippon Nyukazai Co., Ltd., the structure is a sodium polycarboxylate), to an aqueous coating composition containing a water soluble fluorinated polymer wherein at least some of carboxy groups in a fluorinated polymer having carboxy groups are neutralized by a basic compound (Patent Document 4).